According to this post they appear to be date codes. There is a list of date codes here. They are outdated, but you can follow the pattern to figure out when the sprocket was made. the one in your picture appears to be from October/November 2001 if this theory is correct. This site has a more recent list indicating that this trend has continued. The date codes from that page are partially copied below in case those old forums disappear.
This doesn't appear to make sense with codes like aq/ar, but the codes on the sprocket shown most likely indicate date of manufacturer. AQ/AR may indicate the model of cassette they came from. Shimano documentation says that ar is associated with the CS-HG400 9 speed sprockets. AQ seems to be associated with the
CS-M770 cassette. If you google the specific code you are looking for, such as "shimano code aq" (without quotes) you will most likely find shimano documentation for the part in question.